1. Field of the Invention
The invention relates to a field effect transistor (hereinafter referred to as an "FET") having excellent high speed switching performance. An element integrating these FETs is suitable, for example, as a logical element of a ultrahigh speed computer.
2. Description of the Prior Art
The technical concept of the element of this invention is based upon the fact that the electron mobility (66) can be enhanced by reducing ionized-impurity scattering, that is, scattering that is applied to the electrons inside a semiconductor, and lattice scattering. First, the ionized-impurity scattering can be reduced while maintaining a sufficient electron density by isolating a layer, which is to be doped with an impurity as an electron donor, from a layer which permits the electrons to travel, in accordance with a selective doping method. Second, lattice scattering can be reduced by cooling the element. If GaAs is used as the channel for permitting the electrons to travel, the electron mobility of .mu..apprxeq.40,000 cm.sup.2 V.sup.-1 S.sup.-1 is observed in undoped GaAs which is cooled down to 77.degree. K. after selective doping, whereas electron mobility is .mu.=3,000 cm.sup.2 V.sup.-1 S.sup.-1 (300 .degree. K.) in n-GaAs which contains the electron donor by itself (n.apprxeq.1.times.10.sup.18 ion/cm.sup.3). High speed switching elements utilizing this large electron mobility have heretofore been proposed, as exemplified by T. Mimura et al. "Japanese Journal of Applied Physics", Vol. 19, L 225, 1980.
FIG. 1 is a sectional view of a semiconductor device on which the present invention is based. In the drawing, reference numeral 1 denotes a GaAs semi-insulating substrate; 2 is a GaAs layer that does not contain an impurity such as an electron donor; 4 is a Ga.sub.0.7 Al.sub.0.3 As to which an n-type impurity is doped (n=2.times.10.sup.18 ion/cm.sup.3); and 5, 6 and 7 are source, gate and drain electrodes, respectively.
Reference numeral 3 denotes electrons that are fed from the n-Ga.sub.0.7 Al.sub.0.3 As layer 2. The current flowing through the source and drain relies upon this electron movement.
FIG. 2 shows the potential distribution of the conduction band at the interface between the layer 2 and the layer 4. A spike-like carrier profile 12 of the conduction band 11 is shown occurring due to the heterojunction. The electrons are accumulated at the portions below the Fermi level and contribute to conduction.